Process for making OHMIC contacts and photovoltaic cell with ohmic contact

ABSTRACT

Ohmic contacts to p-type IIB/VIB semiconductor are obtained by a process which includes the step of depositing a viscous liquid containing a Group IB metal salt on a surface of a semiconductor, substantially free of oxide groups, heating to form a dried layer, removing the dried layer, washing the surface to remove residual by-products and drying the surface.

The present invention relates to the production of ohmic contacts top-type IIB/VIB semiconductor materials.

The production of thin film IIB/VIB semiconductors, e.g. CdTe, byelectrodeposition and their use in photovoltaic cells is disclosed in GB1 532 616.

The use of heat treatment to convert thin films of near intrinsic orn-type semiconductor based on elements of Group IIB of the PeriodicTable and Te to p-type semiconductor is disclosed in U.S. Pat. No.4,388,483.

In order to make photovoltaic cells with good electrical properties itis necessary to make a good ohmic contact to the layer of IIB/VIBsemiconductor. U.S. Pat. No.4,456,530 discloses a method of obtaininggood ohmic contacts on p-type semiconductor by etching the semiconductorfilm surface with an acidic solution, and then etching with a strongbasic solution, and finally depositing a metal layer. The acidic etchmay be an acid solution of an oxidising agent such as the well-knowndichromate etch. The strong basic etch may be hydrazine monohydrate.

We have found that the stability on prolonged light exposure of cellsprovided with ohmic contacts using the method of U.S. Pat. No.4,456,530, i.e. their resistance to ageing, is not always as high as isdesirable and that the electrical characteristics may be impaired asresult of a reduced shunt resistance and increased series resistance.

We have now found an alternative method of providing ohmic contacts tolayers of IIB/VIB semiconductors.

According to the present invention a method of making an article havingan ohmic contact to p-type IIB/VIB semiconductor is characterised by thesuccessive steps of:

depositing a viscous liquid containing a Group IB metal salt on asurface, substantially free of oxide groups, of the layer of p-typesemiconductor,

heating the resulting deposit to form a dried layer,

removing the dried layer,

washing the surface to remove residual by-products, and

drying the surface.

The preferred method of applying the invention is to deposit aconductive layer on the surface which has been treated in accordancewith the invention. An alternative, less preferred, method is to form aconductive layer, and then to deposit a very thin layer of IIB/VIBsemiconductor on the conductive layer. This layer of IIB/VIBsemiconductor is then subjected to the treatment of the presentinvention. Finally a thicker layer of IIB/VIB semiconductor layer isdeposited on the treated thin layer of IIB/VIB semiconductor whichprovides an ohmic contact to the underlying conductive layer.

The semiconductor is a IIB/VIB semiconductor, i.e. a semiconductorcontaining at least one element from Group IIB and at least one elementfrom Group VIB. In this specification references to IIB and VIB arereferences to the Periodic Table of the Elements as appearing in"Advanced Inorganic Chemistry" by Cotton & Wilkinson, 4th Edition, inwhich Group IIB includes Cd, and Group VIB includes Se and Te. Thepreferred semiconductors are compounds of Cd and Te, which may alsocontain Hg, as disclosed in U.S. Pat. No. 4,548,681. In addition tocompounds of Cd,Te, and Hg it is also possible to use CdTe doped withsmall quantities of Cu, Ag, and Au as disclosed in EP 244 963.

The p-type semiconductor layer may be a thin film semiconductor, but mayalso be a single crystal material. It is particularly preferred to applythe process of the present invention to a p-type polycrystalline thinfilm semiconductor obtained by the heat treatment of a layer of IIB/VIBsemiconductor obtained by electrodeposition.

The IIB/VIB semiconductor layer may form part of a multi-layer articleintended to be formed into a photovoltaic cell. Such an article may beobtained by depositing CdS on a transparent conducting substrate (forexample using electrodeposition or electroless or vacuum deposition),and then electrodepositing the IIB/VIB semiconductor on the CdS layer.

In order to obtain the p-type semiconductor required by the process ofthe present invention the electrodeposited material is heat-treatedusing known methods to form p-type material. If the heat treatment iscarried out in the presence of a gas containing oxygen such as air thena surface layer containing oxides of Cd and Te is produced. Thisoxide-containing layer is undesirable and if it is formed thesemiconductor is subjected to a treatment to remove it. Thus theoxide-containing surface layer may be removed by a treatment with, forexample, a liquid containing a compound which forms a soluble complex orsalt with Cd and a salt with Te.

A particularly preferred complexing compound is diaminoethane. Thediaminoethane is preferably used diluted, preferably in the form of anaqueous solution. The concentration of diaminoethane in the aqueoussolution is preferably in the range 25% to 75% by volume. Anothercompound which may be used is EDTA (ethylene diamine tetraacetic acid)used as its sodium salt. This may be used as an aqueous solution, forexample a 0.1M solution. The pH is preferably 7 or above, e.g. 12.

The oxide removal treatment may be carried out by dipping the IIB/VIBsemiconductor layer into a bath containing the treatment liquid. Thetreatment step may be carried out over a moderately wide range oftemperatures e g. 10° C. to 50° C. The optimum duration of the treatmentstep is dependent on the concentration of complexing compound in thetreatment bath and the temperature. At 20°-25° C. and a diaminoethaneconcentration of 50% wt by volume in an aqueous solution it mayconveniently be in the range 0.5-10 minutes, but more preferably 1-5minutes.

After any step of removing the oxide-containing surface layer which maybe required the viscous liquid containing a Group IB metal salt isdeposited on the treated surface. The Group IB metal salt may be acopper salt e.g. a cuprous halide. Among specific salts which may beused are cuprous chloride, cuprous iodide or cupric sulphate.

The viscous liquid may be a liquid containing finely divided solidparticles to give it the consistency of a paste.

The paste containing the salt of a Group IB metal salt preferablycontains finely divided insulating material in a non-aqueous liquid as athickening agent. Suitable material for mixing with the salt of theGroup IB metal is commercially available as dielectric paste orscreenable masking paste. This dielectric paste, or screen printablemasking paste, is used in the production of printed circuits to maskareas of the board on which the printed circuit is formed. One suchcommercially available dielectric paste is believed to comprisecolloidal clay in a non-conducting liquid. The materials used must ofcourse be inert to the semiconductor on which they are deposited.

The paste may be formed by the addition of a solution of the metal saltin a non-aqueous liquid for example dimethyl formamide to a pre-formedpaste such as the dielectric paste mentioned above. The concentration ofthe salt in the non-conducting liquid may for example be in the range0.1-100 mM. This solution may be mixed with a thickened non-aqueousliquid such as a dielectric paste in a volume/mass ratio in the range100:1 to 1:100, e.g. 10:1 to 1:10, where volume and mass have the samerelationship as liters and kilograms.

Alternatively, the viscous liquid may contain a dissolved viscosifyingagent such as a non-conducting polymer. An example of a polymer whichcan be used is polyvinyl pyrrolidone (PVP).

The viscous liquid may be applied to the semiconductor surface by anyconvenient method e.g. painting with a brush, screen printing, or byspinning, application with a flexible doctor blade, immersion, orspraying to produce a continuous coating on the semiconductor.

The thickness of the viscous liquid layer is not believed to be criticaland may for example be 0.1 to 100 micrometers, e.g. 1 to 100micrometers.

The article which now has a surface layer containing Group IB salt, e.g.cuprous iodide, is subjected to a heat treatment. The heat treatment canbe carried out in an inert atmosphere, e.g. nitrogen or argon, or in airor in vacuum (e.g. during or after the application of the paste). Thetemperature is preferably in the range 50° to 500° C., more preferably150° to 250° C. The duration of the heat treatment will depend on thetemperature but may for example be in the range 1 to 60 minutes.

After the heat treatment step the etched and heat treated layer issubjected to a cleaning step to remove the deposit left by the viscousliquid, e.g. dried paste where a paste is used. Thus dried paste may beremoved by immersing the semiconductor surface in a liquid for examplebutyl acetate and then subjecting the liquid to ultrasonic vibration for1-10 minutes at 10° to 50° C.

After the removal of the deposited material the semiconductor surface iswashed. It is believed that this removes residual by-products e.g. Cdsalts. The washing step is carried out with solvents such as methanol ordeionized water.

The washing step may be carried out at ambient temperature.

After the washing step the semiconductor layer is dried preferably in aninert atmosphere e.g. by passing a stream of nitrogen atmosphere overit.

A conductive contact is deposited on the treated semiconductor surface.Techniques for depositing conductive contacts are well-known and thereis therefore no need to describe these in detail. The conductive contactmay for example be Ni, Au, Te, multilayers of two or more metals (e.g.Ni/Cr or Ni/Al), carbon (e.g. graphite/polymer blends), multilayers ofcarbon with one or more metals, (e.g. Ni/Al) or transparent conductiveoxides, e.g. SnO₂ (TO) or In₂ O₃ :Sn (ITO).

The invention will now be described by reference to the followingexperiments in which experiments identified by number are Examples ofthe invention and experiments identified by letter are comparative testsnot according to the invention.

Example 1

A photovoltaic cell was prepared as follows.

A thin film of CdS was chemically deposited on a glass substrate coatedwith SnO₂ by immersing the glass substrate in a warm alkaline solutioncontaining a cadmium complex ([Cd(NH₃)₄ ]²⁺) and thiourea. The method isdisclosed in N. R. Pavaskar,C. A. Menezes, A. P. Sinha, J.Electrochemical Soc. 124 (1967) pp 743. The deposited film was rinsedwith DIW (deionized water) and dried with nitrogen. The glass substratewith the deposited CdS layer was then heated at 400° C. in air for 10minutes.

The glass substrate was immersed in glacial acetic acid as an etchant toremove surface layers on the CdS. In place of glacial acetic acidhydrazine hydrate would have been equally satisfactory.

A thin film of CdTe was electrodeposited over the CdS layer. Conditionsfor electrodepositing CdTe are disclosed in U.S. Pat. Nos. 4,440,244 and4,456,630 except that the Te ions were added as TeO₂.

The bath electrolyte was an aqueous solution containing 0.5M Cd²⁺, 50ppm Te, 300 ppm Cl⁻, and pH about 1.7. Te ions were added by theaddition of TeO₂ powder. The bath temperature was 70° C.

The electrode potential corrected for resistive losses was held at -0.5volts relative to the Ag/AgCl reference electrode.

The bath was agitated during the electrodeposition of CdTe on the plate,which took place at a plating current density of about 0.14 mA/cm². Thedeposition continued for about 4.5 hours.

The glass substrate, now carrying CdS and CdTe layers, was then heattreated as disclosed in U.S. Pat. No. 4,388,483 to change theconductivity type of the CdTe to p-type.

The multi-layer structure obtained as above was immersed in a vesselcontaining 500 ml of an aqueous solution of diaminoethane (50%diaminoethane by volume in deionized water) at room temperature (about20° C.) for 2 minutes in order to remove oxide layers from the CdTelayer.

The surface was then washed with deionized water and then dried with astream of nitrogen.

A paste was then painted on the treated semi-conductor surface. Thepaste was prepared by mixing a commercially available dielectric pastewith a solution of CuI in dimethyl formamide solution (15 mM) in a ratioof 2 g of dielectric paste to 5 ml of CuI solution.

The coated surface was then heated at 180° C. in nitrogen for 35 minutesand then allowed to cool to room temperature.

The multi-layer structure was then immersed in a bath of butyl acetateand ultrasonic vibration was applied to the bath. This removed thepaste.

Residual solvent was removed from the structure by washing withdeionized water, and blowing dry with nitrogen.

A back contact was then formed on the structure by painting the topsurface with a carbon paste to form dots each having an area of 3 mm² toform individual photovoltaic cells. Carbon pastes are commerciallyavailable for depositing conducting paths on printed circuits.

The performance of these individual photovoltaic cells formed bydeposition of the back contact was measured at room temperature, underan illumination of 100 mW cm⁻² using an ELH type quartz halogen lightsource with dichroic reflector.

The efficiency of a 3 mm² cell was 11.6% both before and aftercontinuous light soak for 288 hours at an illumination intensity of 100mW cm⁻² at 70° C.

Comparative Test A

A comparative test was carried using a layered structure of CdS/CdTeproduced by electrodeposition of CdTe on CdS and heat treatment, as inExample 1. Instead of the novel treatment of Example 1 the CdTe layerwas subjected to an oxidising etch and reducing etch as disclosed inU.S. Pat No. 4,456,630. The etching conditions used were as follows.

The article obtained by depositing CdS and CdTe on the glass coated withtin oxide substrate was dipped in a weak Br₂ /methanol solution for 5secs and then rinsed in deionized water, and dried with nitrogen. It wasthen immersed in an oxidising etch which was a 0.24M solution of amixture of Na₂ Cr₂ O₇.2.5 H₂ O and H₂ SO₄.H₂ O in a ratio of 3:4 byvolume, for less than 1 sec. It was then rinsed with deionized water anddried in a stream of nitrogen It was then dipped into a hydrazinemonohydrate (60%) solution, rinsed in water and then blown dry with N₂.A Cu (3 nm thick) contact followed by a Ni (250 nm) back contact wasthen formed by vacuum evaporation.

The efficiencies obtained from 10 cells (3 mm²) surface area were 9.2%before light soak of 300 hours (90 mW cm⁻², 65° C.) and 7.5% after lightsoak.

We claim:
 1. The process of making an article having an ohmic contact top-type IIB/VIB semi-conductor layer is characterised by the successivesteps of: depositing a viscous liquid containing a Group IB metal salton a surface, substantially free of oxide groups, of the layer of thep-type semi-conductor, heating the resulting deposit to form a driedlayer, removing the dried layer, washing the surface to remove residualby-products, and drying the surface, resulting in an ohmic contact tothe p-type IIB/VIB semi-conductor.
 2. The process according to claim 1wherein a conductive layer is deposited on the surface having said ohmiccontact.
 3. The process according to claim 1 wherein the semiconductoris a compound of Cd and Te.
 4. The process according to claim 1 whereinthe p-type IIB/VIB semiconductor is a thin film.
 5. The processaccording to claim 1 wherein the Group IB metal salt is a copper salt.6. The process according to claim 1 wherein the copper salt is cuprousiodide or cupric sulphate.
 7. The process according to claim 1 whereinthe viscous liquid is a paste containing a Group IB salt a finelydivided insulating material in a non-aqueous liquid as a thickeningagent.
 8. The process according to claim 7 wherein the paste is preparedby adding a solution of Group IB metal salt in a non-aqueous liquid to adielectric paste.
 9. The process according to claim 8 wherein theconcentration of Group IB metal in the non-aqueous liquid is in therange 0.1-100 mM.
 10. The process according to claim 8 wherein thesolution of Group IB metal salt is mixed with thickened non-conductingliquid in a ratio of volume of said solution measured in liters to massof said liquid measured in kilograms in the range of 100:1 to 1:100. 11.The process according to claim 1 wherein the thickness of the viscousliquid deposited on the IIB/VIB semiconductor is in the range 0.1 to 100micrometers.
 12. The process according to claim 1 wherein the heating ofthe viscous liquid on the surface of the Group IIB/VIB semiconductor iscarried out at a temperature in the range 50° C. to 500° C.
 13. Theprocess according to claim 12 wherein the temperature is in the range150° C. to 250° C.
 14. The process according to claim 1 wherein theheating step is carried out for a time in the range 1 to 60 minutes. 15.The process according to claim 1 wherein the deposit left afterdeposition of the viscous liquid and heating the resulting deposit isremoved by bringing the deposit into contact with a non-aqueous liquidand applying ultrasonic agitation.
 16. The process according to claim 1wherein the p-type IIB/VIB semiconductor is brought into contact with aliquid which forms a soluble complex or salt with Cd and salt of Tebefore depositing the viscous liquid.
 17. The process according to claim16 wherein the IIB/VIB semiconductor is brought into contact with anaqueous solution of diaminoethane or a salt of ethylene diaminetetraacetic acid before depositing the viscous liquid.
 18. A processaccording to claim 3 wherein the viscous liquid is a paste containing acopper salt and finely divided insulating material in a non-aqueousliquid as a thickening agent and the heating of the viscous liquid onthe surface of the semiconductor is at a temperature in the range of 50°C. to 500° C. to form a dried layer.
 19. A process according to claim 18wherein before depositing a viscous liquid, the semi-conductor isbrought into contact with a liquid, which forms a soluble complex orsalt with Cd and a salt of Te, and after the depositing and heating, thedried layer is removed by bringing into contact with a non-aqueousliquid and applying ultrasonic agitation.
 20. A process as claimed inclaim 1 wherein said viscous liquid is a dielectric paste.
 21. A processas claimed in claim 20 wherein the Group 1B metal salt is a copper salt.22. A process as claimed in claim 20 wherein the deposit left afterdeposition of the dielectric paste and heating the resulting deposit isremoved by bringing the deposit into contact with a non-aqueous liquidand applying ultrasonic agitation.
 23. A thin film photovoltaic devicecomprising.(a) a transparent conducting layer (b) a CdS layer (c) ap-type IIB/VIB semi-conductor layer (d) an ohmic contact to the IIB/VIBsemi-conductor layer,wherein the ohmic contact is obtained by thesuccessive steps of: depositing a viscous liquid containing a Group 1Bmetal salt on a surface, substantially free of oxide groups of the layerof the p-type semi-conductor; heating the resulting deposit to form adried layer; removing the dried layer; washing the surface to removeresidual by-products, and drying the surface.